This invention is generally related to clock recovery techniques for video and audio applications, and more particularly to phase and frequency synchronization across a packet-based data network that features isochronous data delivery.
An important capability required of a network for media (video and audio) applications is the ability to deliver real time or isochronous video and audio. This isochronous or xe2x80x9cconstant timexe2x80x9d delivery ensures that the media data arrives at known, reliable and predictable, e.g. constant, time intervals. Computer data networking technologies have recently emerged as new vehicles for delivering media. One such technology is known as the High Speed Serial Bus (HSSB), defined in the Institute of Electrical and Electronics Engineers (IEEE) 1394 specification. In the HSSB network, a talker sends the audio or image data in packets, one packet per isochronous transaction. Each such transaction may last no longer than a fixed xe2x80x9ccycle timexe2x80x9d of 125 microseconds during which the packet, containing the audio or image data as its payload, must be delivered to a listener. This technology is referred to as a xe2x80x9cstreaming methodxe2x80x9d by the Society of Motion Picture and Television Engineers (SMPTE) for exchanging television (video and audio) program material. The HSSB with its isochronous mode of operation in which a constant rate of data transfer is guaranteed between a talker node and a listener node allows a versatile and cost effective digital media network to be built.
In addition to the ability to transmit and receive streams of video and audio in a predictable manner, a versatile media network suitable for television program production and editing also needs to synchronize the playback of the media at the listener nodes. This allows the playback of video at a remote location to be precisely controlled, so that the video will start and end at the same time in both local and remote locations of a production facility. Transporting such video content over a network built using the HSSB, however, presents a particularly difficult problem because the HSSB does not provide a versatile synchronization mechanism.
In a HSSB network, both the talker and the listener can process packets synchronously, ie. at the same frequency and phase. Each node has a crystal-controlled reference clock of 24.576 MHz and a 8 kHz cycle time reference clock. A digital cycle time counter tracks the duration of an isochronous transaction and is run by the 24.576 MHz clock. The counter is reset every 125 microseconds by the 8 kHz clock. The network has a packet exchange mechanism for synchronizing the digital cycle time counter of a number of listener nodes to that of a talker node, thus achieving both phase and frequency lock for keeping track of isochronous transactions.
Conventional professional and broadcast video defined by the SMPTE may use a 27.000 MHz video reference clock to finely synchronize the start time of each 30 millisecond video frame. However, the 24.576 MHz reference clock of the HSSB and the conventional broadcast video reference clock of 27.000 MHz have different frequencies. Also, there are not an integer number of HSSB reference clock periods in conventional metrics of broadcast video such as a frame (e.g. 30 msec) or a line (e.g. 63.5 microsec). This makes the synchronization of conventional broadcast video using the time references available in the HSSB network an exceedingly difficult circuit design and manufacturing problem.
A method for recovering clock signals is disclosed. The method includes generating a media sync signal to synchronize processing of digital media, and generating a transmission reference clock signal to define a duration of a transaction through a packet-based data network. The media sync and transmission clock signals may have different frequency and phase. The media is sent to a slave node of the network. The media sync and transmission clock signals are correlated to generate phase correlation information, and the phase correlation information is also sent to the slave node. Accordingly, a relatively low cost and reliable clock recovery technique suitable for synchronizing media streams across a packet-based data network is disclosed.